The present invention generally relates to a heater and more particularly, to a method of assembling a cylindrical heater employing a positive temperature coefficient thermistor element.
Recently, a positive temperature coefficient thermistor element has been employed in a heater of small size and with high reliability, for example, a cylindrical heater or the like.
Referring first to FIG. 1, there is shown an example of a conventional cylindrical heater H1 having a molded cylindrical case 14 of an insulating material, and a pair of positive temperature coefficient thermistor elements 12 accommodated in the cylindrical case 14 and each provided with a pair of electrodes at its opposite surfaces, with each of the thermistor elements 12 being connectively soldered in parallel relation with each other by a pair of lead wires 16. The above mentioned cylindrical heater H1 is assembled in such a manner that the thermistor elements 12 soldered with the lead wires 16 are inserted into the molded cylindrical case 14 and thereafter, a material having excellent thermal conductivity and superior electrical insulating characteristics is filled in a space defined by the cylindrical case 14, the thermistor elements 12 and the lead wires 16.
In FIGS. 2a and 2b, there is illustrated another example of a conventional cylndrical heater H2 comprising a cylndrical case 14a, a positive temperature coefficient thermistor element 12a sandwiched between a pair of terminal strips (not shown) and a pair of elastic heat sinks 17 each having a cross section of a semi-circular shape. The cylindrical heater H2 of the above described type is assembled employing a method wherein an inner assembly is firstly formed in a shape like a rectangular flat plate by wrapping the thermistor element 12a held between a pair of terminal strips into an insulating film 15 and is subsequently inserted into the cylindrical case 14a, with the pair of heat sinks 17 being held therebetween.
There is shown in FIGS. 3a and 3b, a further example of a conventional cylindrical heater H3 having a cylindrical case 14b composed of an elastomer with insulating characteristics and superior thermal conductivity, and a positive temperature coefficient thermistor element 12b held between a pair of terminal strips 18. FIG. 4a shows a cross sectionof the cylindrical case 14b of the cylindrical heater H3 ad FIG. 4b shows a cross section of an inner assembly of the cylndrical heater H3 wherein the thermistor element 12b is sandwiched between a pair of terminal strips 18. The cylindrical heater H3 of the above described type is assembled by a method wherein the inner assembly is forcibly inserted into an opening 19 formed in the cylindrical case 14b and having a rectangular cross section as shown in FIG. 4a. In this example, an inner short side length D1 of the opening 19 is smaller than a total thickness D2 of the inenr assembly at a stage before insertion of the inner assembly into the cylndrical case 14b, and the positive temperature coefficient thermistor element 12b and a pair of terminal strips 18 are applied with pressure to each other under the influence of elasticity of the cylindrical case 14b after insertion of the inner assembly into the cylindrical case 14b.
However, in the assembling method as explained with reference to FIG. 1, since the positive temperature coefficient thermistor elements 12 are directly soldered to the lead wires 16, the assembly steps are complex and have the drawback that the thermistor elements 12 are exposed to be deteriorated by heat when being soldered with the lead wires 16. Furthermore, since a filler is completely filled in a space around the thermistor elements 12, there has been another drawback that the thermistor elements 12 are exposed to be deteriorated in quality thereof under the influence of lack of oxygen. In addition, the cylindrical heaters H1 have been manufactured undesirably at high cost, since a time consuming assembling work is required for ensuring reliability of products.
Meanwhile, in the assembling method as stated with reference to FIGS. 2a and 2b, there has been a disadvantage related to high production cost, since an increased number of parts is required and consequently, the assembling process is prolonged due to the fact that it is necessary to arrange the heat sinks 17 and the terminal strips individually. Moreover, there has been a drawback that in respect of reliability, such problems as the lack of dielectric strength or the like are liable to arise from breakage of the insulating film 15 or the like.
Furthermore, in the assembling method as described with reference to FIGS. 3a, 3b and 4a, 4b, although the problems as described above can be generally settled, since large amount of pressure is required when the inner assembly having the positive temperature coefficient thermistor element 12b sandwiched between a pair of terminal strips 18 is to be forcibly inserted into the opening 19 of the cylindrical case 14b, such problems as deformation of the terminal strips 18, damage to the thermistor element 12b or the like are frequently invovled. There has occasionally been an extreme case wherein the cylndrical case 14b is damaged when the inner assembly is inserted thereinto and it has been difficult to achieve a desirable work, since the inner assembly has a necessity to be forcibly inserted into the cylndrical case 14b. In addition, the temperature has occasionally varied on the surface of the cylindrical case 14b, since an extremely thin air layer is produced between the cylindrical case 14b and the terminal strips 18 by a deflection of an inside surface of the opening 19 of the cylndrical case 14b, which deflection is caused by the forced insertion of the inner assembly into the cylindrical case 14b.